The Etesians: from observations to reanalysis

The Etesians are among the most persistent regional scale wind systems in the lower troposphere that blow over the Aegean Sea during the extended summer season. In this study we evaluate the performance of three different reanalysis products (the twentieth century reanalysis, 20CR; the 40-year European Centre for Medium-Range Weather Forecasts, ECMWF, Re-Analysis, ERA40; and the recently released ECMWF reanalysis ERA-20C) in capturing the Etesian wind system. Three-hourly data from 24 stations over Greece are used and compared with reanalysis outputs for the extended summer season (May–September) from 1971 to 2000. An objective classification of Etesians based on the pressure difference over the Aegean is provided. Classified Etesian days are then investigated as well as the associated large scale atmospheric circulation. Results highlight the ability of the investigated reanalyses to adequately describe the Etesian meteorological regimes. Intense Etesians are associated with stronger geopotential height anomalies over western-central Europe and the Eastern Mediterranean and with pronounced changes in the mean position of the jet streams. Finally, station time series provide evidence for less frequent intense Etesian days at the end of the extended summer season.